1. Field of the Invention
The present invention relates to a circuit breaker for protecting an electric circuit and electric load equipments by automatically breaking the circuit at the time of occurrence of an overload or a short circuit, and more particularly, to a contactor assembly for such a circuit breaker.
2. Description of the Background Art
A mold cased circuit breaker (MCCB) is installed at an electric power distributing board among several electric power systems of a factory and a building. The circuit breaker serves as a switch for supplying or cutting off electric power source to a load side under no load state, and cuts off power supplied to a load side from a power side in order to protect a circuit and electric load equipments when a great abnormal current due to a short-circuit or a ground fault flows on a circuit under a load state.
FIG. 1 is a sectional view showing an inner construction of a circuit breaker in accordance with the conventional art, and FIG. 2 is a frontal view of a movable contactor assembly of the circuit breaker in accordance with the conventional art, which shows a closed circuit state, and FIG. 3 is a frontal view of the movable contactor assembly of the circuit breaker in accordance with the conventional art, which shows an opened circuit state.
As shown, a circuit breaker 1 comprises a mold case 10, fixed contactors 9 and 9′ installed in the mold case 10 with a predetermined distance, a movable contactor assembly 17 rotatably disposed between the fixed contactors 9 and 9′, a trip mechanism 11 for tripping the circuit breaker by detecting a large current, a switching mechanism 12 automatically operated by the trip mechanism 11 or manually operated by operating a switching handle 1, for separating a movable contactor 4 from the fixed contactors 9 and 9′ thereby cutting off a circuit, and an arc extinguishing mechanism 13 for extinguishing arc gas of a high temperature and a high pressure generated between a contact 4b of the movable contactor 4 and contacts 9b and 9b′ of the fixed contactors 9 and 9′ at the time of switching a circuit.
The mold case 10 is provided with the above mechanisms therein, and is formed of an insulating material to insulate the mechanisms of phases, and to prevent foreign materials such as dust from being introduced into the mold case 10.
The fixed contactors, that is, a fixed contactor 9 for connecting a power source and a fixed contactor 9′ for connecting an electric load are respectively provided with a contact 9b and a contact 9b′ at the end thereof. The movable contactor 4 is provided with a contact 4b at both ends thereof.
The movable contactor assembly 17 comprises a movable contactor 4 rotatably positioned between the fixed contactors 9 and 9′ for maintaining a closed state or an opened state, a rotation shaft 2 for supporting the movable contactor 4 rotatably, and a pair of springs 15 and 16 respectively having one ends 15a and 16a fixed to the movable contactor 4 and the other ends 15b and 16b fixed to the rotation shaft 2 for elastically rotating the movable contactor 4 centering around a virtual rotation axis 2a by an electromagnetic repulsive force generated at the contacts 9b and 9b′ when a large current flows on a circuit due to a short-circuit or a ground fault. The method for supporting the movable contactor 4 to the rotation shaft 2 centering around the virtual rotation shaft 2a is called as a self centering.
As shown in FIG. 2, a state that the contact 4b of the movable contactor 4 is in contact with the contacts 9b and 9b′ of the fixed contactors 9 and 9′ is called as ‘a closed circuit state’. As shown in FIG. 3, a state that the contact 4b of the movable contactor 4 is separated from the contacts 9b and 9b′ of the fixed contactors 9 and 9′ is called as ‘an opened circuit state’. Also, converting the closed circuit state to the opened circuit state is called as ‘separating and opening’.
The movable contactor 4 is supported by the pair of springs 15 and 16 disposed to be symmetrical to each other centering around the virtual rotation axis 2a. 
One ends 15a and 16a of the springs 15 and 16 are fixed to the movable contactor 4, and the other ends 15b and 16b thereof are fixed to the rotation shaft 2. Accordingly, as shown in FIG. 2, when a normal current flows on a circuit, the contact 4b of the movable contactor 4 is in contact with the contacts 9b and 9b′ of the fixed contactors 9 and 9′ thereby to maintain a closed circuit state. Under the state, the springs 15 and 16 provide an elastic force to the movable contactor 4 so that the movable contactor 4 can be maintained in contact with the fixed contactors 9 and 9′. Accordingly, an electric current flows from the fixed contactor 9 to the fixed contactor 9′ through the movable contactor 4.
As shown in FIG. 3, when a large current flows on a circuit due to a short-circuit or a ground fault, the movable contactor 4 is separated from the fixed contactors 9 and 9′ by an electromagnetic repulsive force between the contact 4b of the movable contactor 4 and the contacts 9b and 9b′ of the fixed contactors 9 and 9′ thereby to have a rotation moment. Accordingly, the movable contactor 4 overcomes an elastic force of the springs 15 and 16, and rotates in a clockwise direction thereby to cut off the circuit. An unexplained reference numeral 18 designates a stopper for limiting a rotation range of the movable contactor 4.
The conventional movable contactor assembly of a circuit breaker has the following problems. When the movable contactor 4 is separated from the fixed contactors 9 and 9′, the virtual rotation axis 2a of the movable contactor 4 is not stable, so it cause to generate a fluctuation of the movable contactor 4 in right and left directions and up and down directions. Also, when the movable contactor 4 is separated from the fixed contactors 9 and 9′, an elastic restoration force of the springs 15 and 16 increases and thereby the movable contactor 4 becomes in contact with the fixed contactors 9 and 9′ again due to the restoration force. That causes a re-contact between the contacts at the time of a short-circuit and a re-separation by an electromagnetic repulsive force, thereby continuously generating an arc. Accordingly, an instant current limiting characteristic is not obtained and severe damages may be caused to the circuit breaker and the load devices.